1. Field of the Invention
The invention relates to an ink droplet ejecting method and apparatus of an ink jet type.
2. Description of Related Art
According to a known ink jet printer of an ink jet type, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material. When the ink flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle. However, when the ink flow path volume increases, the ink is introduced into the ink flow path from an ink inlet. In this type of printing head, multiple ink chambers are formed by partition walls of a piezoelectric ceramic material. An ink supply device, such as ink categories, are connected to one end of each of the multiple ink chambers. The opposite end of each of the ink chambers is provided with an ink ejecting nozzle (hereinafter referred to simply as “nozzles”). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character of a figure.
An example of this type of ink jet printer is a drop-on-demand type ink jet printer that ejects ink droplets, which is popular because of a high ejection efficiency and a low running cost. An example of a drop-on-demand type ink jet printer is a shear mode type that uses a piezoelectric material, which is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051.
As shown in FIGS. 7(a) and 7(b), this type of ink droplet ejecting apparatus 600 includes a bottom wall 601, a top wall 602 and shear mode actuator walls 603 (shown in FIG. 8 as 603a-g) located therebetween. The actuator walls 603 each include a lower wall 607 bonded to the bottom wall 601 and polarized in the direction of arrow 611, and an upper wall 605 formed of a piezoelectric material, the upper wall 605 being bonded to the top wall 602 and polarized in the direction of arrow 609. Adjacent actuator walls 603, as a pair, define ink chamber 613 (shown in FIG. 8 as 613a-d) therebetween. The actuator walls 603 that are adjacent to the ink chamber, in a pair, define a space 615 which is narrower than the ink chamber 613.
A nozzle plate 617 having nozzles 618 (shown in FIG. 8 as 618a-d) is fixed to one end of each of the ink chambers 613, while the opposite end of each of the ink chambers is connected to an ink supply source (not shown). Electrodes 619 (shown in FIG. 8 as 619a-d) and 621 are respectively formed on both side faces of each actuator wall 603, as metallized layers. More specifically, electrode 619 is formed on the actuator wall 602 on the side of the ink chamber 613, while electrode 621 is formed on the actuator wall 603 on the side of the space 615. The surface of electrode 619 is covered with an insulating layer 630 for insulation from ink. Electrode 621, which faces the space 615, is connected to a ground 623, and electrode 619, which is provided in each ink chamber 613, is connected to a controller 625, which provides an actuator drive signal to the electrode.
The one-way propagation time T is a time required for the pressure wave in the ink chamber 613 to propagate longitudinally through the same chamber. Given that the length of the ink chamber 613 is L and the velocity of sound in the ink present in the ink chamber 613 is a, the time T is determined to be T=L/a.
According to the theory of pressure wave propagation, upon lapse of time T, or an odd-multiple time thereof, after the above application of voltage, the internal pressure of the ink chamber 613 reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode in the ink chamber 613c is returned to 0(V). As a result, the actuator walls 603e and 603f revert to their original state (FIGS. 7(a) and 7(b) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure, and the pressure developed by the reverting of the actuator walls 603e and 603f to their original state before the deformation, are added together to provide a relatively high pressure in the vicinity of the nozzle 618c in the ink chamber 613c, whereby an ink droplet is ejected from the nozzle 618c. An ink supply passage 626, shown in FIG. 7(b), that communicates with each of the ink chambers 613, is formed by members 627 and 628.
Conventionally, in this type of ink droplet ejecting apparatus 600, when an ink droplet of a small volume is to be ejected for enhancing the printing resolution, a control has been provided to decrease the driving voltage in multiple steps, for example. However, such a method of controlling the voltage in multiple steps leads to an increase in cost of a driver IC, etc., and attempting to reduce the volume of an ink droplet gives rise to the problem that even the speed of the ink droplet decreases. In order to obtain an ink droplet of a small volume without decreasing the ink droplet speed, it has been proposed to use in additional pulse of a low voltage level, after application of a jet pulse and before completion of ink ejection. However, this proposal also leads to an increase in cost of a driver IC, etc. because multiple voltages are needed as driving pulses.